Nowadays there are a number of optic interferometry techniques and instruments for measuring geometric sizes such as distance, surface form, dimensions, movement and vibration. The most common technique for vibration analysis is based on so-called laser doppler velocimetry (LDV). Using this technique the vibrations are measured at a single point such that the movement of the measured object on this single point results in wavelength shifts in the incident light reflected from the point. The shift in wavelength is given from the amplitude and frequency of the object. Using measuring systems based on LDV, one may scan the beam over an object area to get information about the object vibrations over a complete range.
Another well-known method is holographic interferometry based on illumination of the entire object using an expanded laser beam. The reflected laser light illuminates a light sensitive glass plate (hologram) together with a reference beam coherent to the object light. An interference pattern is registered in the hologram, and after developing one may recreate an image of the object superimposed, showing so-called interference fringes, providing information about the movements of the object. Such recording of vibration is called time averaging recording, because the light sensitive plate is exposed over a time period which is equal to or larger (often far larger) than the vibration period of the object. There is also an electronic version of holographic interferometry, where the glass plate is replaced by a video camera. This technique is known as ESPI (Electronic Speckle Pattern Interferometry) or TV-holography [1], [2].
Using TV-holography, the vibration is presented as a video image of the object with fringes indicating the amplitude distribution of the vibration. There are also numeric versions of TV-holography. One of these uses pulsed illumination to give quantitative and numeric information about the vibration amplitude distribution and phase distribution of the object [3].
When interferometry measurements occur on objects with surfaces, resulting in diffuse reflection of the incident light, the measurement is usually called a speckle interferometry measurement. The denotation “speckle” is referring to coherent light, such as laser light, getting a granulated and irregular nature after reflection from a diffusely reflecting surface.
Measuring movement on specular surfaces may be made by simpler interferometry setups, where for example a light beam from a laser is divided in two by a beam divider. The first light wave is sent in towards the object being measured, the other towards an ordinary mirror. The two reflected light waves are combined again and are superimposing each other, and the light waves are captured by a detector or a detector array which also may register the interference term arising from the superimposition of the two waves. When the object is moving, the intensity in the interference term will be modulated, and thereby give information about the movements of the object.
There are also other variants of interferometers, for example interferometers based on white light or other low coherent light sources. Such systems are commonly used in combination with microscopic rendering for measuring the surface form (the topography) of microscopic objects. Using low coherent sources, interference between the two interfering light waves is achieved only when the object light and the reference light are travelling the same distance after the splitting in the beam divider. As an example, by moving the object in a direction towards or away from the beam divider, distances may be registered at which interference arises in the different parts of the surface, and in this manner find the surface topography of the object being investigated.
The principle of all the aforementioned methods have clear, mutual characteristics, as all of them are based on interferometry between two or more light waves.